Test other flash video sites google video weebls-stuff etc to see if it IS just youtube. If so, then it's probably a problem at their end. Try different quality levels and player settings. Also try adding &fmt=18 to the end of the youtube videos URL. Finally check the volume isn't turned down on the youtube video (sorry to patronize)
If the other sites don't work either then it's not youtube's fault, so don't bother with the previous things. Check your volume in the bottom right hand corner- double click it to see if one channel is turned up (e.g. CD-player) but one isn't (e.g. wav). What browser are you using. If IE then right click and start without addons, see if that works. If so, then it's something Integrated that's breaking it. You could try and fix it, but I'd just recommend getting firefox. If you DO have firefox, then disable any addons you have and see if that helps.
If you STILL have problems, then re-install falsh player and arestart your computer... That's all I can think of hope it helps
